1. Field of the Invention
This invention relates to passive athermalization of a lens system.
2. Brief Description of the Prior Art
A perennial problem for imaging infrared optical systems has been thermal defocus. The serious effects of varying temperatures upon the behavior of optical systems have been a matter of concern to optical designers and users for many years. Largely because of the high refractive index change with temperature (dn/dT) of the common infrared materials, the overall power of most infrared systems is very sensitive to temperature.
The first serious treatment of the effect of temperature on optical performance was made by J. W. Perry, Proceedings of the Physical Society, 55, 257 (1943). Perry separated the thermal effects into five groups, these being, (1) variation in optical path length because of: (a) thermal expansion of the optical materials, (b) thermal change in the index of refraction, (c) thermal expansion of the mechanical mountings, (d) internal strain in optical materials caused by thermal changes in the materials themselves or their mechanical mounts, (2) low thermal conductivity of optical materials resulting in slow internal equalization of temperature, (3) atmospheric effects such as convection, inversions or other density variations in the media surrounding the system, (4) thermal variation in the transmission of spectrally selective components, such as narrowband optical filters and (5) thermal variation of the aberrational corrections.
With the exception of internal strain caused by thermal changes, which can be controlled by appropriate design, choice of materials and construction, and thermal variations in the transmission of spectrally selective components, the disturbing phenomena can be resolved into two broad classes, these being (A) homogeneous (i.e., those distributions which involve a variation of the mean temperature of the optical system with time, but where spatial variations within the system from the mean are negligible) and (B) heterogeneous (i.e., those distributions which arise from a spatial variation of temperature throughout the optical system, possibly varying with time but having a fixed mean value). These two broad classes are realized independently only under laboratory conditions. All naturally occurring phenomena are a combination of both homogeneous and heterogeneous effects. Nevertheless, many systems approach conditions where one or the other class becomes dominant.
The perceptible effects of a homogeneous temperature change are principally a shift in the position and size of the lens image. The heterogeneous distribution in general results in a loss of definition in the image which cannot be recovered by a simple focus shift.
The above considerations are equally valid for systems operating in the infrared region of the spectrum. For fixed-focus optical systems, the focus shift typical of a homogeneous temperature distribution is the major problem. The effects of the heterogeneous temperature distribution, while present, are swamped in the gross loss of modulation caused by simple thermal defocus. For typical infrared systems, the most significant factors in this shift are (1) thermal change in the index of refraction of the glass, (2) thermal expansion of the optical materials and (3) thermal expansion of the mechanical mounts. Thermal change of index of refraction is by far the most important effect in most infrared systems. As an example, the refractive index change with temperature (dn/dT) of germanium, the most common optical material used in the infrared spectral range, is approximately 400.times.10.sup.-6 /.degree. C. For comparison, the dn/dT of the common optical glass BK7 is 1.6.times.10.sup.-6 /.degree. C. at 5461 .ANG. for 20.degree. to 40.degree. C. according to the Schott Optical Glass book. The index change with temperature for germanium is significantly (about 250 times) greater than for the visible optical glass. Furthermore, germanium is not unique in this respect among the candidate materials in the infrared spectral region.
The prior art methods of correction available to athermalize infrared optical systems are well known and can be broken into three general methods, these being (1) thermal correction wherein the optical system may be held at a constant temperature independent of the external environment, a number of existing systems, both infrared and visible having used this method in constructing a thermostatically controlled oven around the optical system; (2) thermo-mechanical correction wherein the expansion and contraction of various mechanical members of the optical system can be arranged to compensate for the image plane shift caused by thermo-optical effects, many systems having also used this method, some more successfully than others, some examples of this technique being the use of bimetallic belleville washers around the periphery of the lenses and/or focal planes and other systems having used thermal sensors such as thermocouples to sense the temperature of the optical system and then used this information to drive a motor that attempts to restore focus in an open-loop manner using previously calculated movements of lenses and/or focal planes; and (3) thermo-optical correction wherein the basic optical design can be planned with the intention of controlling thermal aberrations, this technique being totally passive and, if properly done, reducing the sensitivity of the system to both homogeneous and heterogeneous temperature effects.
The basic technique of thermo-optical correction is quite similar to the well-known techniques of controlling chromatic aberrations. However, the limited material choices available in the infrared region have made the simple theory difficult to apply in practice.
In a paper prepared by Robert Gibbons entitled "Athermal Infrared Optics", dated February 1976, the contents of which are incorporated herein by reference, passive athermalization of a doublet lens is described whereby two materials are chosen such that the ratio of the Abbe v-number and equivalent thermal v-number are the same for both materials. With this, a solution that corrects for chromatic aberration also maintains the focal length of the doublet with temperature changes. This is a one solution setup limited by choice of materials. If it is desirable to design the thermal correction, for example, to increase the focal length with temperature at the same rate as the expansion of the lens housing to maintain focus, the chromatic correction would be compromised. This concept does not have the ability to independently design thermal correction separate from chromatic correction.
U.S. Pat. No. 4,679,891 of Michael Roberts describes optical athermalization using a three lens solution. The first two lenses are from materials whose refractive indices are relatively temperature insensitive, one positive and one negative, which accomplishes most of the chromatic and spherical correction. The third lens is a negative lens made from a material whose refractive indices are relatively temperature sensitive and is of lower dispersion and higher refractive index than the first two lenses which accomplishes most of the thermal correction. Germanium is the only material known that will work well for the third lens in the 8 to 12 micron wave band. If germanium should be unsuitable for use because of its thermal absorption properties, this scheme could not be used because an alternate material for the third lens is not known. In the 3 to 5 micron wave band, germanium is very dispersive and will not function for the third lens. The materials available for 3 to 5 micron use are not as well suited for this concept. There is no known low dispersion material for this wave band, rather only some lower than others. As a result, there would be compromising or a push-pull of thermal correction vs. chromatic correction.